<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    * {
        margin: 0;
        padding: 0;
    }

    li {
        list-style-type: none;
    }

    a {
        text-decoration: none;
        font-size: 14px;
    }

    .nav {
        margin: 100px;
    }

    .nav>li {
        position: relative;
        float: left;
        width: 80px;
        height: 41px;
        text-align: center;
    }

    .nav li a {
        display: block;
        width: 100%;
        height: 100%;
        line-height: 41px;
        color: #333;
    }

    .nav>li>a:hover {
        background-color: #eee;
    }

    .nav ul {
        display: none;
        position: absolute;
        top: 41px;
        left: 0;
        width: 100%;
        border-left: 1px solid #FECC5B;
        border-right: 1px solid #FECC5B;
    }

    .nav ul li {
        border-bottom: 1px solid #FECC5B;
    }

    .nav ul li a:hover {
        background-color: #FFF5DA;
    }
</style>
</head>

<body>
    <ul class="nav">
        <li>
            <a href="#">微博</a>
            <ul>
                <li>
                    <a href="">私信</a>
                </li>
                <li>
                    <a href="">评论</a>
                </li>
                <li>
                    <a href="">@我</a>
                </li>
            </ul>
        </li>
        <li>
            <a href="#">微博</a>
            <ul>
                <li>
                    <a href="">私信</a>
                </li>
                <li>
                    <a href="">评论</a>
                </li>
                <li>
                    <a href="">@我</a>
                </li>
            </ul>
        </li>
        <li>
            <a href="#">微博</a>
            <ul>
                <li>
                    <a href="">私信</a>
                </li>
                <li>
                    <a href="">评论</a>
                </li>
                <li>
                    <a href="">@我</a>
                </li>
            </ul>
        </li>
        <li>
            <a href="#">微博</a>
            <ul>
                <li>
                    <a href="">私信</a>
                </li>
                <li>
                    <a href="">评论</a>
                </li>
                <li>
                    <a href="">@我</a>
                </li>
            </ul>
        </li>
    </ul>
    <script>
        var nav = document.querySelector('.nav')
        var lis = nav.children;   //得到四个小li
        for (var i = 0; i < lis.length; i++) {
            lis[i].onmouseover = function () {
                //鼠标经过当前这个li  就把里面的第二个孩子ul。显示出来 
                //用的是 子节点children[1]   拿到第二个孩子
                this.children[1].style.display = 'block'
            }
            lis[i].onmouseout = function () {
                this.children[1].style.display = 'none'
            }
        }

    </script>
</body>

</html>